An important application of the present invention is in vacuum cleaners. Such machines include apparatus for applying suction to dislodge undesirable particulate matter from a surface to be cleaned, by generating a high velocity air flow. The suction apparatus includes structure for channelling the dirt-laden air into a narrow stream. A collection bag or other receptacle is mounted to receive the particle and air flow. A typical bag includes a jacket formed of air pervious material, such as paper and/or tightly woven fabric, to mechanically filter particulate matter, while allowing the filtered air to dissipate outwardly through the bag and back into the external environment.
Vacuum cleaners which rely solely on mechanical filtration, however, filter only particles of greater than a given size, while allowing smaller particles to pass through the filter and re-enter the external environment. This is because, in order to permit the air to pass freely out of the bag, the interstices in the paper or fabric, which permit air to pass through, cannot be too small. Otherwise, the suction air stream is inhibited, and air velocity becomes too low for good suction. While one could increase suction and air volume by use of more powerful electric motor drive systems, the use of inordinately large and heavy electric motors in a household appliance such a vacuum cleaner can become both impractical and uneconomical. The weight and cost of large motors make their use prohibitive in vacuum cleaners designed for household use.
The fine particles that pass through the bag and back into the external environment can include very small dust particles, contributing to odor and re-accumulation. Other particles escaping filtration are allergy-aggravating pollen and bacteria, as well as mites, which can be a health hazard.
One proposal to improve a vacuum cleaner's effectiveness in filtering very small particles has been to add on-board electrostatic filtration equipment, while still maintaining a reasonable pressure drop through the filter media and hence reducing the size and power of the suction motor system. Such equipment has included at least two elements between which an electrical potential difference is applied. The electrical potential difference generates an electric field between the elements. It also causes the elements to become electrically charged. The element to which voltage of a given polarity is applied attracts oppositely charged particles of dirt, as well as oppositely charged, naturally occurring ions, such as gas ions.
The elements are positioned in the particle-laden air stream. A charged element, as noted above, attracts oppositely charged particles passing along in the air stream. Moreover, even some neutrally charged particles are attracted to the element by a phenomenon known as dielectrophoresis.
It has also been proposed to augment such electrostatic filtration by provision of a so-called "corona" device in the air stream. A corona device produces an electrical space charge which is distributed generally throughout a region. Such space charge, if generated in the particle-laden air stream, pre-charges the particles. This imposition of charge on the particle increases the force attracting or repelling them to the electrically polarized filter element.
One problem with on-board vacuum cleaner electrostatic filters is the necessity for providing a relatively high electrical voltage on a substantially continuous basis while the machine is operating. This often requires large, heavy and expensive power supplies, sometimes including heavy batteries. Such equipment degrades portability and ease of machine operation.
A further proposal has been to place in the air stream a piece of electrically charged fleece.
Another type of device for electrostatic filtering incorporates what is known as "electret" material. Electret materials have low electrical conductivity and usually have dielectric properties as well. They also have the property of retaining charge polarization for a long time. Electret materials have been used as electrostatic filters in surgical masks.
The filter equipment described above has a further disadvantage. When a charged surface "loads up" with accumulated particles, the charge on the charged filter element can become neutralized or canceled, due to the opposite polarization of particles and ions attracted to its surfaces. This tends to cancel the generated electrical fields, hindering or totally disabling operation of the device.
An object of this invention is to provide electrostatic filtering apparatus and circuitry (1) whose effectiveness does not deteriorate as the amount of retained filtered material increases, (2) which is effective at low operating voltages, and (3) which is lightweight, relatively inexpensive and compact.